According to English Nature, the UK government's advisory body on conservation,
an extensive study of genetically modified (GM) herbicide tolerant oilseed
rape crops in Canada has revealed that genes from separate GM varieties
can accumulate ('gene stacking') in plants that grow from seed spilled
at harvest (volunteer plants). This happens because different varieties
cross-pollinate, and their offspring may contain the accumulated genes
from GM varieties with different genetic traits. In Canada these plants
are now resistant to several widely used herbicides, with farmers regularly
resorting to old herbicides to control them. In effect, they are on
the road to becoming nuisance weeds.

The Canadian system of voluntary guidelines advising farmers to leave
a separation distance of 175 m between different GM varieties seems
to have broken down, and 'gene stacking' is now widespread in Canada.
A code of practice for farmers growing GM crops in the UK has already
been developed by the industry body SCIMAC.

Dr Brian Johnson, English Nature's biotechnology advisor said, "Our
report shows that the SCIMAC code is probably inadequate to prevent
gene stacking happening in Britain, if these crops were commercialized.
The consequences for farmers could be that volunteer crops would be
harder to control and they might have to use different, and more environmentally
damaging, herbicides to control them."

English Nature are concerned that attempts to eliminate GM volunteers
with multiple herbicide tolerance in 'weedy' crops like oilseed rape
could lead to more intensive herbicide use in field margins and uncropped
habitats, which can be important refuges for wildlife.

Dr. Johnson said, "We do not yet know how 'stacked gene' plants
would behave either in farmers' fields or in the wild. The European
regulatory system has not yet approved GM herbicide tolerant oilseeds
for general release. English Nature will be working with DEFRA and ACRE
to ensure that risks from possible gene stacking are properly addressed,
and that we avoid the mistakes that have been made in Canada."

The European Commission has recently proposed that a threshold of up
to 0.7% GM seed should be allowed in batches of conventional crop seed.
English Nature are deeply concerned that if this proposal were to be
adopted, it might be a recipe for gene stacking, because the GM plants
from a seed batch could be made up of several varieties that would inevitably
hybridise, giving 'volunteer' plants next season with multiple GM traits.
It will be difficult to police seed batches to ensure that this does
not happen.

English Nature has been pressing the GM industry to explain how to deal
with these issues before GM crops are released widely, rather than wait
for stacking to emerge and then try to control the rogue crop plants.